Continuous-wave (CW) modelocked lasers are important for the generation of ultrashort light pulses. One such laser, a colliding pulse modelocked laser, is a stable source of subpicosecond light pulses. Ultrashort light pulses are used to study ultrafast phenomena with techniques ranging from spectroscopy to electrooptic sampling.
Various types of noise are known to accompany the ultrashort light pulses from CW modelocked lasers. Noise for such lasers has been characterized as amplitude noise and phase noise noise. See U. Keller et al., IEEE J. of Quant. Elect., Vol. 25, No. 3, pp. 280-288 (1989) and D. von der Linde et al., Appl. Phys. B, 39, pp. 201-217 (1986). Amplitude noise gives rise to measurement fluctuation within the signal detection bandwidth while limiting one's ability to achieve high sensitivity in the experimental technique. High sensitivity is generally understood to mean a sensitivity limited only by shot noise processes. Phase noise also adds noise to measurements while degrading temporal resolution of the experimental technique.
For optical probing experiments in which an optical probe beam is modulated in proportion to a variable under measurement, it is important to understand the amplitude spectrum of the laser generating the optical probe beam. Understanding of the amplitude spectrum has been limited in the prior art to merely characterizing the spectrum without uncovering any source noise appearing in the spectrum. Techniques have been devised to avoid or ameliorate the effects of noise in the amplitude spectrum. For example, reduction of the detection bandwidth or translation of the detected signal to higher frequencies by chopping techniques.
Amplitude noise causes fluctuations in optical intensity of the probe beam. Intensity fluctuations within the detection bandwidth directly cause corresponding fluctuations of the variable being measured which, in turn, degrade the signal-to-noise ratio of the measurement. By reducing the detection bandwidth, it is possible to reduce the amount of amplitude noise entering the measuring apparatus. Unfortunately, when applied to optical probing experiments, this procedure generally results in unacceptable acquisition times which are susceptible to long term drift of the measured variable.
In order to overcome the latter problem, it has been proposed to translate the detected signal to higher frequencies by chopping techniques because amplitude noise of most lasers decreases with increasing frequency. Residual amplitude noise at the chopping frequency reduces sensitivity by limiting the minimum detectable signal for a given acquisition time. When amplitude noise peaks appear in the power spectrum, standard procedures have evolved to keep the chopping frequency away from the noise peaks. In some lasers such as the colliding pulse modelocked laser, the noise peaks occur at regular intervals rising 40 dB or more above background noise levels and having widths of several hundred kHz.
While the prior art has developed a characterization of amplitude noise, an understanding of its deleterious effects, and several techniques for performing experiments somewhat effectively in the presence of the amplitude noise, there has been no effort to locate a source of the amplitude noise. Moreover, there has been no known technique or apparatus developed for eliminating or even substantially reducing the level of the amplitude noise.